A solenoid I suppose does not need this kind of isolation
Well this is where electronics is more of an art than a science, their is no real way to determine in advance how much decoupling is needed in any one situation. Most engineers err on the risk averse side and would rather put too much decoupling in than too little. A lot depends on the sort of electromagnetic environment it finds itself in.
There are minimum levels of noise immunity a product must reach to receive formal approval for one standard or another, they are all slightly different. But environments exist that have much more interference than these values.